High brightness broadband optical frequency comb sources have many applications in medicine, spectroscopy, microscopy, ranging, sensing and metrology. Such sources need to be highly robust, have long term stability, and also comprise a minimal component count with a high degree of optical integration for mass market applications. Especially, broadband optical frequency comb sources based on passively modelocked lasers in conjunction with frequency broadening or supercontinuum generation in highly nonlinear fibers or waveguides have generated of lot of interest. Particularly when used in conjunction with short pulse fiber lasers, an all-fiber system construction is possible for supercontinuum generation which results in benefits such as greatly simplified manufacturing routines, low cost and high levels of thermo-mechanical stability.
Fiber based supercontinuum sources can produce spectral output from the UV to the mid-IR and have attracted a vast amount of research in the last few years, see for example J. M. Dudley et al., ‘Supercontinuum generation in optical fibers’, Cambridge University Press (2010). To reach the mid-IR, for example the wavelength range from about 2.5-10.0 μm, soft glasses or heavy metal oxide glasses may be implemented for supercontinuum generation, as recently reviewed by J. H. V. Price et al., ‘Supercontinuum generation and nonlinearity in soft glass fibers’, in chapter VI of J. M. Dudley et al., ‘Supercontinuum generation in optical fibers’, Cambridge University Press (2010). Such fiber based mid-IR sources operating in the mid-IR can potentially replace more established optical parametric oscillators (OPOs), amplifiers (OPAs) and generators (OPGs) and are therefore very actively pursued.
Optical fiber frequency combs are conveniently constructed from mode locked lasers by controlling both the repetition rate as well as the carrier envelope offset frequency (CEO) inside the laser resonator, as for example disclosed in U.S. Pat. No. 6,785,303 to Holzwarth et al. The repetition rate of a resonator can be modulated at MHz repetition rates using piezo-electric transducers or electro-optic transducers as well known in the state of the art. In contrast, in '303 it is suggested to use much slower modulation mechanisms for the control of the CEO frequency, for example, modulation of the optical pump power coupled into the laser resonator.
Methods for rapid control of the CEO frequency were disclosed in U.S. patent application pub. No. 2010/0195677, ('677) ‘Pulsed Laser Sources’, to Fermann et al., where incorporation of an addressable intra-cavity component was suggested for carrier phase control. For example, various configurations included rapid control of the pressure of a fiber Bragg grating, control of fiber Bragg grating temperature, and/or multiple feedback loops to decouple any dependency between the repetition rate and the CEO frequency. Other techniques to introduce controllable phase variation were also suggested.
Rapid control of the CEO frequency was further suggested in U.S. patent application pub. no. 2010/0225897, entitled ‘Optical scanning and imaging systems based on dual pulsed laser systems’, to Fermann et al., via the control of the intra-cavity loss inside a mode locked laser resonator. In particular the incorporation of an acousto-optic modulator enables the control of the CEO frequency at MHz modulation rates, many orders of magnitude faster than possible with optical pump modulation. The contents of the '677 and '897 applications are hereby incorporated by reference in their entirety.
Passively mode locked lasers generally require an intra-cavity saturable absorber to favor short pulse operation over cw operation, where additional design criteria have to be met to suppress Q-switching instabilities. Recently saturable absorbers based on graphene to facilitate modelocking have been suggested in J. M. Dawlaty et al., Probing Ultrafast Dynamics of Electrons and Holes in Graphene, Optical Society of America Conf. on Lasers and Electro-Optics, paper CFU7 (2008). Graphene based saturable absorbers can be directly deposited on optical gain materials such as optical fibers as discussed in A. Martinez et al., Optical Deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing, Opt. Express, vol. 18, pp. 23054 (2010) and are therefore of great interest.
However, to date, passively mode locked lasers and optical frequency combs or in particularly passively mode locked fiber lasers and fiber based frequency combs are still relatively difficult to manufacture, have a large component count and their high cost prevents their use in mass applications.